The invention relates to a pilot stage for pressure control valves. More particularly, it relates to a pilot stage or valve for pressure control valves having a valve chamber connected to a first control channel, a closure element axially displaceable in the valve chamber and engaging therein a valve seat, and a pressure-setting spring associated with the closure element so as to exert a closing force on the closing element in the direction of the valve seat.
The function of pressure control valves in hydraulic systems is to limit the system pressure to a specific pre-set pressure level. If this pre-set value is achieved, then the pressure control valve responds and returns the surplus volume flow (i.e. the difference between the pump flow rate and the load flow rate) to the tank. In the case of a relatively large flow rate, the pressure control valves are provided with pilot stages.
Pilot stages for pressure control valves may include manual adjusting means for the pressure-setting spring. If such pilot stages are combined with a controllable directional valve, the operation of the assembly may be changed from a pressure control mode to a pressureless circulation mode. This change from one mode to another is effected by bypassing the pilot stage via the directional valve. Undesirable pressure peaks in the hydraulic system are caused when the main stage response is too rapid to this change in operation from one mode to another.
Pilot stages for pressure control valves may also include proportional actuating coils for adjusting the pressure. Because of the limited actuation force of an actuating coil, the free cross section of the valve seat (or area of the closing element exposed to the pressure in the valve seat channel) must be smaller in proportional pilot stages, than in manually adjustable pilot stages. An undesirable pressure increase is caused in the hydraulic system with an increasing flow rate through the main stage when a valve seat is used which has a relatively small cross section.
Proportional pressure control valves often include an additional spring-loaded, manually-adjustable, pressure release valve. The object of an additional pressure release valve is to safeguard the hydraulic system from excessive pressures caused by high control currents at the proportional actuating coil. This additional pressure release valve entails increased expense and requires additional space.
The disadvantages and problems of the prior art, pilot-controlled, pressure control valves will be discussed below and are illustrated in FIGS. 1 and 2. In both figures, a pressure control valve 10 is shown. This valve 10 comprises a main piston 12, a main valve seat 13, a main piston spring 14 and a main control chamber 16. The object of the pressure control valve 10 is to protect a hydraulic system 18 against inadmissible high pressures and/or to maintain an adjusted pre-set pressure setting. To that end, the hydraulic system 18 is connected through the pressure control valve 10 to a pressureless tank 22 by means of a relief channel 20.
The reference number 24 indicates a pilot valve with manual pressure adjustment. The valve has a valve seat 26 and a closure cone 28. The latter is axially displaceable in a valve chamber 30 and may sealingly engage the valve seat 26. A manually adjustable pressure-setting spring 32 is associated with the closure cone 28 so that it exerts thereon a closing force in the direction of the valve seat 26. The valve chamber 30 is connected to the tank 22 by way of a first control channel 34. A second control channel 36 opens into the valve seat 26. The other end of the second control channel 36 is hydraulically connected by way of a third control channel 38 to the main control chamber 16 and by way of a fourth control channel 40 to the hydraulic system 18.
If the system pressure P.sub.E in the hydraulic system 18 is lower than the opening pressure set at the pressure-setting spring 32, then the pilot valve 24 is closed and the main control chamber 16 is pressurized with the system pressure P.sub.E. Since the main piston 12 has in the main control chamber 16 a main control surface or pilot area 42 which corresponds approximately to the free cross section of its valve seat 13, the main piston 12 is then in hydrostatic equilibrium and is pressed by way of the main piston spring 14 into its valve seat 13 to form a seal. Consequently the connection between the hydraulic system 18 and the tank 22 through the relief channel 20 is closed.
If the system pressure P.sub.E in the hydraulic system 18 exceeds the opening pressure set at the pressure-setting spring 32, the closure cone 28 opens against the pressure-setting spring 32 and enables pilot oil to be discharged through the valve chamber 30 and the first control channel 34 to the tank 22. Admission of pilot oil through the fourth control channel 40 from the hydraulic system 18 is limited by a first throttle 44. A relatively constant control pressure is established in the second control channel 36 and the main control chamber 16. If the system pressure P.sub.E continues to rise, then ultimately the main piston 12 opens and the hydraulic system 18 is connected through the relief channel 20 to the tank 22, and as a result the system pressure is limited.
In FIG. 1 the hydraulic system 18 may be depressurized using an electromagnetically operated directional valve 46 (e.g. a 2/2-way valve with spring return) which is responsive to a control signal. This directional valve 46 is connected hydraulically in parallel with the pilot valve 24 and thereby bypasses the pilot valve 24. When the directional valve 46 is opened electromagnetically, the main control chamber 16 is relieved to tank 22. In other words, by bypassing the pilot valve 24, the pressure adjustment function becomes ineffective. The main control piston 12 opens, because its main control surface 42 is now relieved of pressure, and the flow in the hydraulic system 18 is deviated to the relief channel 20. The system pressure that establishes in the hydraulic system 18 is dependent on the counter-force of the main piston spring 14 and the throttles 44, 48, 50 arranged in the pilot system.
The prior art depressurization of the hydraulic system 18 of FIG. 1 provides an unsatisfactory result. The main piston 12 responds too abruptly to the bypassing of the pilot valve 24 and opens within a few milliseconds, so that a relief shock occurs, corresponding to the rapid flow rate increase in relief channel 20. In addition to causing noise, such relief shocks may lead to disturbances in the system, such as damage to filters, coolers, seals, and supporting elements, for example, or to undesirable vibrations in the machines connected to the system. Throttle 48 (inserted in the outlet of the main control chamber 16) and throttle 50 (inserted in the tank channel of the pilot valve 24) provide an inadequate slowing down of the opening movement of the main piston 12. The effectiveness of the throttles 48, 50 are limited because they must have a minimum free cross section so that dirt particles are prevented from blocking them.
The prior art pressure control valve 10 in FIG. 2 is used as a pilot-controlled, proportional, pressure control valve having an independent mechanical pressure relief. The reference number 52 indicates a pilot valve which, like the pilot valve 24, comprises a valve seat 54 and a closure cone 56. This closure cone 56 is axially displaceable in a valve chamber 58 and sealingly engages in the valve seat 54. In place of the manually adjustable pressure-setting spring 32 in the pilot valve 24, the pilot valve 52 has an electrically-controllable, proportional, actuating coil 60 generating the closing force of the closure cone 56 in proportion to an electric control current 62. Since the closure force acting on the closure cone 56 determines the opening pressure of the pilot valve 52 and consequently the pressure in the main control chamber 16 of the pressure control valve 10, the pressure P.sub.E in the hydraulic system 18 is continuously varied in proportion to the electrical control current 62. In an alternative embodiment, the proportional actuating coil 60 acts on a pressure spring (not shown) which exerts a closing force on the closure cone 56 in the direction of the valve seat 54 and continuously controls the bias thereof, and thus the pressure P.sub.E in the hydraulic system 18.
Both embodiments have the disadvantage that the free cross section of the valve seat 54 has to be designed relatively small because of the limited actuating force (generally about 60N to 100N) of the proportional actuating coil 60. Therefore the free cross section of the valve seat 54 is normally about seven to ten times smaller than the free cross section of the valve seat 26 in the manually adjustable pilot valve 24. This considerably reduced flow cross section of the valve seat 54 leads to parasitic pressure losses in the pilot valve 52, which is undesirable since main control chamber 16 is now far more dependent on fluctuations in the flow in control channel 40. Fluctuations in the flow in the control channel 40 may be caused by the following. A flow increase through pressure control valve 10 results in higher flow forces acting on the main piston 12 in the closing direction, This causes the pressure P.sub.E in the hydraulic system to increase, which in turn causes an increased flow rate in control channel 40. The effect is a rise of the system pressure P.sub.E with an increasing flow rate Q through the pressure control valve 10. The larger the pressure loss in pilot control valve 52, the steeper the rise in the characteristic curve (Q; P.sub.E) of the pilot-controlled pressure control valve 10. While actuating coils provide benefits over manually adjustable valves, the problems (discussed above) caused by the smaller free cross sectional area point out the need for an improve pilot stage for a pressure control valve.
In FIG. 2 a manually adjustable pilot valve 24' is arranged in parallel with the pilot valve 52. The object of pilot valve 24' is to protect the hydraulic system 18 against excessive pressures. For example, high pressures may be caused by a defective control of proportional actuating coil 60 (e.g. excessive control current 62). This pilot valve 24' corresponds in function and construction with the pilot valve 24 of FIG. 1 and has a valve seat 26', a closure cone 28', a valve chamber 30' and a pressure-setting spring 32'. Incorporating a second valve 24' into the pilot part of the pressure control valve 10 entails increased expense and requires additional space.
The foregoing demonstrates that there is a need for a pilot stage for a pressure control valve that protects the hydraulic system from the relief shocks and pressure losses associated with the prior art pilot stages.